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Gas in the digestive Tract

Everyone has gas and eliminates it by burping or passing it through the rectum. However, many people think they have too much gas when they really have normal amounts. Most people produce about 0.6 to 2.0 litres per day and pass gas about 14 times per day.

Gas is made primarily of odourless vapours – carbon dioxide, oxygen, nitrogen, hydrogen and sometimes methane. The unpleasant odour of flatulence comes from bacteria in the large intestine that release small amounts of gases that contain sulphur.

Although having gas is common, it can be uncomfortable and embarrassing. Understanding ways to reduce symptoms and treatment will help most people find some relief.

What causes gas?

Gas in the digestive tract (oesophagus, stomach, small intestine and large intestine) comes from two sources;
  • Swallowed air
  • Normal breakdown of certain undigested foods by harmless bacteria naturally in the large intestine (colon)
Swallowed air

Air swallowing (aerophagia) is a common cause of gas in the stomach. Everyone swallows small amounts of air when eating and drinking. However, eating or drinking rapidly, chewing gum, smoking or wearing loose dentures can cause some people to take in more air.

Burping or belching is the way most swallowed air – which contains nitrogen, oxygen and carbon dioxide – leaves the stomach. The remaining gas moves into the small intestine, where it is partially absorbed.

A small amount travels into the large intestine for release through the rectum. (The stomach also releases carbon dioxide when stomach acid and bicarbonate mix, but most of this gas is absorbed into the blood stream and does not enter the large intestine).

Breakdown of undigested foods
The body does not digest and absorb some carbohydrates (the sugar, starches and fibre found in many foods) in the small intestine because of a shortage or absence of certain enzymes. This undigested food then passes from the small intestine into the large intestine, where harmless and normal bacteria break down the food, producing hydrogen, carbon dioxide and, in about one third of all people, methane. Eventually these gases exit through the rectum.

People who make methane do not necessarily pass more gas or have unique symptoms. A person who produces methane will have stools that consistently float in water. Research has not shown why some people produce methane and others do not.

Foods that produce gas in one person may not cause gas in another. Some common bacteria in the large intestine can destroy the hydrogen that other bacteria produce. The balance of the two types of bacteria may explain why some people have more gas than others do.

Which foods cause gas?

Most foods that contain carbohydrates can cause gas. By contrast, fats and proteins cause little gas.

The sugars that cause gas are raffinose, lactose, fructose and sorbitol.

Beans contain large amounts of this complex sugar. Smaller amounts are found in cabbage, brussel sprouts, broccoli, asparagus, other vegetables and whole grains.

Lactose is the natural sugar in milk. It is also found in mild products, such as cheese and ice cream, and processed foods such as bread, cereal and salad dressing. Many people, particularly those of African, Middle Eastern and Asian background have low levels of the enzyme lactase needed to digest lactose. Also, as people age, their enzyme levels decrease. As a result, over time people experience increasing amounts of gas after eating food containing lactose.

Fructose is naturally present in onions, artichokes, pears and wheat. It is also used as a sweetener in some soft drinks and fruit drinks.

Sorbitol is a sugar found naturally in fruits, including apples, pears, peaches and prunes. It is also used as an artificial sweetener in many diabetic foods and sugar free candies and gums.

Most starches, including potatoes, corn, noodles and wheat produce gas as they are broken down in the large intestine. Rice is the only starch that does not cause gas.

Many foods contain soluble and insoluble fibre. Soluble fibre dissolves easily in water and takes on a soft, gel-like texture in the intestines. Found in oat bran, peas and most fruits, soluble fibre is not broken down until it reaches the large intestine, where digestion causes gas.

An insoluble fibre, on the other hand, passes essentially unchanged through the intestines and produces little gas. Wheat bran and some vegetables contain this kind of fibre.

What are some symptoms and problems of gas?

The most common symptoms of gas are belching, flatulence, abdominal bloating and abdominal pain. However, not everyone experiences those symptoms. The determining factors, probably are how much gas the body produces, how many fatty acids the body absorbs, and a person's sensitivity to gas in the large intestine. Chronic symptoms caused by too much gas or by a serious are rare.

An occasional belch during or after meals is normal and releases gas when the stomach is full of food. However, people who belch frequently may be swallowing too much air and releasing it before the air enters the stomach. Sometimes a person with chronic belching may have an upper GI disorder, such as peptic ulcer disease, gastro-oesophageal reflux disease (GORD), or gastritis.

Believing that swallowing air and releasing it will relieve the discomfort of these disorders, this person may unintentionally develop a habitual cycle of belching and discomfort. Frequently, the pain continues or worsens, leading the person to believe they may have a serious disorder.

Two rare chronic gas syndromes are associated with belching: Megablase syndrome and gas bloating syndrome. The Megablase syndrome, which causes chronic belching, is characterised by severe air swallowing and an enlarged bubble of gas in the stomach following heavy meals. The resulting fullness and shortness of breath may mimic a heart attack. Gas bloating syndrome may occur after surgery to correct GORD. The surgery creates a one-way valve between the oesophagus and stomach that allows food and gas to enter the stomach, but often prevents normal belching and the ability to vomit.

Another common complaint is passage of too much gas through the rectum (flatulence). However, most people do not realise that passing gas 14 to 23 times a day is normal. Although rare, too much gas may be the result of severe carbohydrate malabsorption or overactive bacteria in the colon.

Abdominal Bloating
Many people believe that too much gas causes abdominal bloating. However, people of complain of bloating from gas often have normal amounts and distribution of gas. They actually may be unusually aware of gas in the digestive tract.

Doctors believe that bloating is usually the result of an intestinal motility disorder, such as IBS. Motility disorders are characterised by abnormal movements and contractions of intestinal muscles. These disorders may give a false sensation of bloating because of increased sensitivity to gas.

Splenic-flexure syndrome is a chronic disorder that seems to be caused by trapped gas at bends (flexure) in the colon. Symptoms include bloating, muscle spasms and upper abdominal discomfort. Splenic-flexure syndrome is often associated with irritable bowel syndrome.

Any disease that causes intestinal obstruction, such as Crohn's disease or colon cancer, may also cause abdominal bloating. In addition, people who have had many operations, adhesions (scar tissue), or internal hernias may experience bloating or pain. Finally, eating a lot of fatty food can delay stomach emptying and cause bloating and discomfort, but not necessarily too much gas.

Abdominal pain and discomfort
Some people have pain when gas is present in the intestine. When gas collects on the left side of the colon, the pain can be confused with heart disease. When it collects on the right side of the colon, the pain may feel like the pain associated with gallstones or appendicitis.

What diagnostic tests are used?

Because gas symptoms may be caused by a serious disorder, those causes should be ruled out. The doctor will take a dietary history and symptoms will be reviewed. The doctor may ask the patient to keep a diary of foods and beverages consumed for a specific time period.

If lactase deficiency is the suspected cause of the gas, the doctor may suggest avoiding mild products for a period of time. A blood or breath test may also be used to diagnosed lactose intolerance.

In addition, to determine if someone produces too much gas in the colon or is unusually sensitive to the passage of normal gas volumes, the doctor may ask patients to count the number of times they pass gas during the day and include this information in a diary. Careful review of diet and the amount of gas passed may help relate specific foods to symptoms and determine the severity of the problem. If a patient complains of bloating, the doctor may examine the abdomen for the sound of fluid movement to rule out ascites (build up of fluid in the abdomen) and for signs of inflammation to rule out diseases of the colon.

The possibility of colon cancer is usually considered in people 50 years of age and older and in those with a family history of colorectal cancer, particularly if they have never had a colon examination (sigmoidoscopy or colonoscopy). These tests may also be appropriate for someone with unexplained weight loss, diarrhoea, or blood in the stool visibly or on testing.

For those with chronic belching, the doctor will look for signs or causes of excessive air swallowing. If needed, a gastroscopy to view the oesophagus, stomach and upper small intestine and to take appropriate biopsies to rule out disease.

How is gas treated?

The most common ways to reduce the discomfort of gas are changing diet, taking medicines and reducing the amount of air swallowed.

Doctors may tell people to eat fewer foods that cause gas. However, for some people this may mean cutting out healthy foods, such as fruits and vegetables, whole grains and milk products. Doctors may also suggest limiting high fat foods to reduce bloating and discomfort. This helps the stomach empty faster, allowing gases to move into the small intestine. Unfortunately, the amount of gas caused by certain foods varies from person to person. Effective dietary changes depend on learning through trial and error how much of the offending foods one can handle.

Non-prescription medicines
Many non-prescription or over-the-counter medicines are available to help reduce symptoms, including antacids with simethicone and activated charcoal. The usual dose of simethicone containing medications is 2 to 4 tablespoons of the medication take 30 minutes to 2 hours after meals. These medications will help gas in the stomach, but not intestinal gas. Activated charcoal tablets may provide relief from gas in the colon or large bowel. Studies have shown that when taken before and after a meal, intestinal gas is greatly reduced. The usual dose is 2 to 4 tablets taken just before eating and 1 hour after meals.

The enzyme lactase is available in liquid and tablet form over-the-counter, adding a few drops of liquid lactase to milk before drinking it, or chewing lactase tablets just before eating lactose-containing foods helps lactase digestion. There is a limit to its effectiveness and a large volume of lactose needs to be avoided if you have lactose intolerance, even with this medication.

Prescription Medicines
Doctors may prescribe medicines to help reduce symptoms, especially for people with a motility disorder. Promotility or prokinetic drugs such as metoclopramide (Maxalon) and Cisapride (Prepulsid) may move gas through the digestive tract quickly.

Reducing Air Swallowed
Methods to reduce air swallowed include avoiding chewing gum and hard sweets. Eating at a slow pace and checking with a dentist to make sure that dentures fit properly should also help. Nervous air swallowing is a difficult habit to break, as after a time one may become unaware of the action.


Although gas may become uncomfortable and embarrassing, it is not life threatening. Understanding causes, way to reduce symptoms and treatment, will help most people find some relief.

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